The invention relates to a process and a system for creating internal stress in a metallic workpiece by laser shock peening (LSP).
Laser shock peening is a process by which internal stress can be created in a metallic workpiece with a pulsed laser beam. Internal stress in the metallic workpiece is often desired because it can prevent fatigue or crack propagation when the workpiece is loaded.
Conventionally, in this case, the material to be treated is acted upon by an ablative surface layer before laser shock peening. The surface layer is formed, for example, by a metal coating, a metal foil or is made of organic materials. When the pulsed laser beam acts upon this surface layer, the surface layer evaporates and is changed into the plasma condition (state of ionization). Simultaneously with the application of the laser pulse, a covering layer is generated over the treatment site, which covering layer is formed, for example, by flowing water. This covering layer contributes to the fact that the transient plasma formed by the effect of the laser beam on the surface layer is spatially fixed for a time period corresponding approximately to the pulse duration. However, this quasi balanced condition is breached by the plasma pressure so that the covering layer can no longer withstand the plasma pressure and the plasma expands freely. In the process, dynamic impulses are transmitted to the surface of the workpiece which are caused by shock waves which, in turn, lead to the induction of internal stress in the workpiece.
In this case, the laser pulse with an energy of, for example, 5 to 50 J is used for the evaporation of the surface layer as well as for the plasma formation. The used laser beams conventionally have a pulse duration of from 10 to 50 nanoseconds (nsecs).
As required and depending on the workpiece configuration, such a laser impulse is applied to the workpiece at various sites in a spatially offset, simultaneous or time-staggered manner in that, for example, the workpiece and the laser generating device are moved relative to one another and, in each case, identical laser impulses act upon various sites on the workpiece.
Thus, a single laser pulse always acts upon one site, which laser pulse is sufficiently energetic (by controlling the laser pulse energy as well as the pulse duration) that it results in the production and propagation of the plasma and that the induced solid-state shock wave has a sufficiently high shock intensity so that the plastic yield point in the workpiece is exceeded and internal stress is thereby created in the metallic workpiece. The process is used mainly for creating compressive internal stress on surface areas, i.e., in depths of up to 10 mm, for protecting against stress crack corrosion but also for the purpose of deformation.
Accordingly, one object of the invention to provide a process and a device system for creating internal stress in a metallic workpiece using laser shock peening (LSP), which process improves the utilization of the energy of the laser beam and thereby results in more targeted and stronger shock waves in the workpiece, which shock waves, in turn, cause the induction of internal stress.